This invention relates to a water-absorbent resin and a process for producing this resin. In detail, it relates to a water-absorbent resin having average particle diameter in a specially defined range, narrow range of particle distribution, and a surface of uniformly improved quality and, in particular, being superior in water absorption capacity, water absorption rate, suction force, and gel strength etc., showing that water absorption properties are in good balance, showing that an amount of elution of water-soluble resin (hereinafter referred to as water-soluble component) is only small, and being very suitable as sanitary materials, and also, a process for producing the water-absorbent resin. Furthermore, this invention relates to a water-absorbent resin of a new, novel type showing angle-lacking, non-sphere, being superior in handling and treating, and having a surface of uniformly improved quality, and a process for producing the water-absorbent resin.
Hitherto, an attempt has been carried out to use a water-absorbent resin as an absorbent sanitary material for absorbing body fluids such as a sanitary cotton, a disposable diaper, and the like. There have been known, as water-absorbent resins for this purpose, a hydrolyzed starch-acrylonitrile graft polymer (Japanese Official Patent Gazette, shouwa No. 49-43395), a neutralized starch-acrylic acid graft polymer (Japanese Official Patent Provisional Publication, shouwa No. 51-125468), a saponified vinyl acetate-acrylic acid ester copolymer (Japanese Official Patent Provisional Publication, shouwa No. 52-14689), a hydrolyzed acrylonitrile or acrylamide copolymer (Japanese Official Patent Gazette, Shouwa No. 53-15959), and crosslinked products of these polymers, a crosslinked product of a partially neutralized polyacrylic acid (Japanese Official Patent Provisional Publication, Shouwa No. 55-84304) and others.
Incidentally, as properties to be wanted for water-absorbent resins, are cited high water absorption capacity, a water absorption rate, and high gel strength of water-contained swelling gel when the resins are coming in contact with aqueous liquid, and superior suction force to suck up water from a basic material containing aqueous liquid. These properties hitherto have been in a poor balance. That is, these properties are not in directly proportional relation, in particular, water absorption capacity and water absorption rate or gel strength and suction force are in reversely proportional relation, so that there has been found a trend that, as the water absorption capacity increases, other properties decrease. When some resins of a high water-absorbent capacity come in contact with aqueous liquid, aqueous liquid does not spread over the whole part of a water-absorbent resin and the resins form lumps, that is, what we call fish-eyes, so that an extreme lowering of a water absorption rate is observed. Also, in a case of that these water-absorbent resins are used for an absorption body of sanitary materials, the above-described water-soluble component being contained in the water-absorbent resins affects on the absorption capacity of an absorption body, liquid-spreading in a absorption body, and so on. Especially, as the water-absorption capacity for a water-absorption resin increases, elution of a water-soluble component increases in amount, so that there has been found a problem that the resin can not properly used as sanitary materials.
As methods to improve the above-described properties with maintaining their good balance, there have been proposed methods to improve such properties as a water absorption rate etc. by crosslinking the surface of an obtained water-absorbent resin, without giving damage for water absorption capacity which the water-absorbent resin itself has. They are a method wherein a water-absorbent resin being dispersed in a hydrophilic organic solvent or a hydrophobic organic solvent in presence of water and undergoing to react in addition of a crosslinking agent (or its aqueous solution) (Japanese Official Patent Gazette, shouwa Nos. 61-48521 and 60-18690) and a method wherein a water-absorbent resin powder was mixed with a crosslinking agent or a liquid composition containing a crosslinking agent to treat with heat (Japanese Official Patent Provisional Publication, shouwa Nos. 58-180233, 59-189103, and 61-16903) and so on.
In these cases, of importance are uniform dispersion of a crosslinking agent over the surface of a water-absorbent resin and proper permeation into a neighborhood of the surface and, in addition, it is liked that the process is of advantage to industry. However, hitherto known methods have had problems in these points. That is, in the method wherein a water-absorbent resin being dispersed in a solvent and undergoing a crosslinking reaction, a large amount of solvent is required and so, its recovery process is of disadvantage to industry. Especially, in a case being carried out in a hydrophobic organic solvent, distribution of a crosslinking agent on the surface of a water-absorbent resin is apt to become ununiform, so that the crosslinking of surface becomes ununiform. In the other hand, the method wherein a water-absorbent resin is mixed with a liquid component containing a crosslinking agent and treated with heat, is of great advantage to industry, and however, in a case of that particle diameter of a water-absorbent resin is small or distribution of particle diameter is broad, there was found a case that, though being affected on a treatment solution mixing with the water-absorbent resin powder, the powder meets together making a large lump (a fish-eye) and so, it is rather hard to crosslink uniformly the surface. Furthermore, though by doing these treatments such properties as water absorption rate and suction force are somewhat improved, but the improvement is still insufficient and, in particular, elution of a water-soluble component could not be prevented. Thus, has not yet found a method sufficiently satisfied in point of that various kinds of properties of a water-absorbent resin are improved maintaining good balance of properties.